This invention pertains to methods and systems for removing noise from signals. The invention finds particular application to telemetry systems used in implantable medical devices such as cardiac pacemakers and implantable cardioverter/defibrillators.
Implantable medical devices, including cardiac rhythm management devices such as pacemakers and implantable cardioverter/defibrillators, typically have the capability to communicate data with a device called an external programmer via a radio-frequency telemetry link. One use of such an external programmer is to program the operating parameters of an implanted medical device. For example, the pacing mode and other operating characteristics of a pacemaker are typically modified after implantation in this manner. Modern implantable devices also include the capability for bidirectional communication so that information can be transmitted to the programmer from the implanted device. Among the data which may typically be telemetered from an implantable device are various operating parameters and physiological data, the latter either collected in real-time or stored from previous monitoring operations.
Noise refers to any unwanted signal that interferes with the transmission and processing of data signals in a communications system. Such noise may arise from sources either internal or external to the system. Because of limited energy storage capability, implantable medical devices must necessarily transmit their data with a low signal energy, making the transmissions very susceptible to interference from noise. This means that an external programmer can only be satisfactorily used to receive data in relatively noise-free environments. Because of the widespread nature of electromagnetic noise sources, such a constraint may not only be inconvenient to the patient and clinician, but could also be hazardous in an emergency situation. Both broadband and narrowband noise sources contribute to the problem, with modem CRT monitors being a particularly common source of narrowband noise.
The present invention relates to a system and method for removing narrowband noise from a received signal. In a particular embodiment, after digitizing the received signal, narrowband noise is removed from the input signal samples with a series of notch filters having center notch frequencies generated adaptively so that the notch frequencies match the frequency peaks of a detected noise spectrum. The noise spectrum is detected by first computing a power spectrum of the input signal and then subtracting from it a template spectrum corresponding to an expected input signal without noise. A template spectrum is computed from a representative input signal generated by receiving a transmitted signal under noise-free conditions so that when it is subtracted from the input signal spectrum, the result approximates the power spectrum of the narrowband noise alone. In order to produce a detected noise spectrum that most closely approximates the true noise spectrum, the template spectrum is scaled by a factor that reduces the total power in the detected noise spectrum to a minimal value. The frequency peaks in the detected noise spectrum are then identified and used to synthesize filters with corresponding notch frequencies to remove the noise from the input signal.
The narrowband noise removal method may be employed in a system and method for receiving telemetry data from an implantable medical device to result in an improved capability for operating in noisy environments. In an exemplary system, the transmitted signal from the implantable device is a radiofrequency carrier waveform modulated with digitally encoded data in the form of transmit pulses. Further noise immunity may be provided to the system by matched filtering of the input signal samples and adaptive pulse detection.